The invention relates to a drive mechanism for a saw chain of a chain saw.
In Swedish Patent Application 9302842-1 there is shown a device for automatically tensioning the saw chain of a chain saw, wherein the chain runs around a loose outer drive ring which is rolling on a drive sprocket of a traditional rim type. The drive ring has an annular row of circumferentially spaced orifices, through which the drive links of the saw chain enter an annular row of circumferentially spaced recesses in the periphery of the drive sprocket. Axial displacement between the drive ring and the drive sprocket are prevented by stop flanges.
To obtain a large enough contact surface between the drive links and the recesses, the drive ring must be made thin, which makes the bridges between the orifices of the drive ring vulnerable and difficult to manufacture. Even with a thin drive ring, there is a tendency that the contact surface of the recess which contacts the leading edge of the drive links becomes too small, which makes the links climb the drive sprocket in a forward direction if the rotational velocity is non-uniform, as is often the case with high speed piston engines. Moreover, if the bridges between the orifices of the drive ring are damaged the drive ring will split and may cause chain damage or other malfunction.